310 



PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 



also in the case of the pines with evergreen needle-shaped leaves, where, as a rule, 

 the water compensating for that exhaled by the leaves cannot come quickly through 

 open channels, but only slowly through the woody cells. Usually the cuticle and 

 cuticular layers are of equal thickness over the whole leaf surface ; this is so espe- 

 cially in smooth, shiny, leathery evergreen leaves. Not infrequently, however, an 

 irregular thickening is seen, particularly in the neighbourhood of stomata, where 

 circular ramparts are raised, as in Protea mellifera (see fig. 67 ^), or peg-shaped pro- 

 jections are formed, as in the Bamboo (see fig. 66), or elongated hair-like filaments 

 arise, as in the rolled leaves of Azalea and many Heaths (see figs. 71 and 72). 



It would, however, be erroneous to suppose that this formation of a thick cuticle 

 on the epidermis is a peculiarity of evergreen leaves. Plants which are surrounded 







73.— TWckened Stratified Cuticle. 



1 Vertical section of a portion of the leaf of Mistletoe (Viscum album); x420. 2 Vertical section of a portion of the leaf 

 ot HoUy {Ilex AquifoUum); x500. 3 Vertical section of leaf of Oleander (J^eWum Oleander); x320. 



all the year by a damp atmosphere, and are never exposed in their natural condition 

 to the danger of too much evaporation, very often have evergreen leaves, and yet 

 the outer wall of the epidermal cells is not at all, or only very slightly, thicker than 

 the inner; and conversely, plants with apparently thin delicate leaves, which are 

 green only in the summer, have quite conspicuous thickening-layers. A knowledge 

 of these conditions is of the utmost importance in plant culture, and gardeners know 

 very well that many plants, although they appear to be capable of resistance, can 

 never be removed from the damp air of the greenhouses, because the leaves then 

 become desiccated like those of aquatic plants which have been taken out of water 

 and exposed to the air. A species of palm, Caryota propinqua, which is repre- 

 sented in its native habitat in fig. 74 opposite, was grown in the botanical gardens 

 at Vienna, and it developed in the damp air a magnificent stem with fine large 

 leaves. On a summer day, when the temperature of the open air coincided with 

 that of the greenhouse, this Caryota, together with the tub in which it was rooted, 

 was carried into the open and placed in a somewhat shady place, but partly exposed 



